In some applications, transistors may be “biased” to operate in a particular fashion. For example, a field effect transistor (FET) may have a “bias current” that continuously flows between two device terminals such as source and drain nodes of the transistor. If the drain-to-source voltage across the transistor changes, the bias current may change.
Circuit designers typically design transistor circuits to satisfy certain constraints such as size, cost, performance, and power consumption. If the bias current changes beyond certain limits due to changes in drain-to-source voltage, the design constraints may not be satisfied.